Deep drawn microcellularly foamed polymeric containers made via solid-state gas impregnation thermoforming

ABSTRACT

The present invention is directed to a deep draw microcellularly foamed polymeric container comprising a polymeric sidewall integrally connected to a polymeric base along a bottom edge. The polymeric sidewall and base are contiguous with each other and define a shape of an open top container. The polymeric sidewall and base have a contiguous inner microcellular foam structure (having average cell diameters ranging from about 5 to about 100 microns) surrounded by a smooth outer skin layer integrally connected therewith. The polymeric sidewall defines a container height and a top opening, wherein the top opening defines a top opening width, and wherein the polymeric base defines a container base width, and wherein the area defined by the top opening is greater than the area defined by the polymeric base, and wherein the ratio of the container height (h) to the top opening width (w) is greater than about 1:1 (h:w).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.11/106,190 filed on Apr. 13, 2005 (now abandoned), which applicationclaims benefit of priority to PCT International Application No.PCT/US2004/015246 filed on May 14, 2004 (converted), which applicationclaims benefit of priority to U.S. Provisional Application No.60/471,477 filed on May 17, 2003, all of which applications areincorporated herein by reference in their entireties for all purposes.

TECHNICAL FIELD

The present invention relates generally to foamed polymeric objects and,more specifically, to deep drawn microcellularly foamed polymericcontainers made via solid-state gas impregnation thermoforming, as wellas to related methods.

BACKGROUND OF THE INVENTION

Microcellular plastic foam refers to a polymer that has been speciallyfoamed so as to create micro-pores or cells (also sometime referred toas bubbles). The common definition includes foams having an average cellsize on the order of 10 microns in diameter, and typically ranging fromabout 0.1 to about 100 microns in diameter. In comparison, conventionalplastic foams typically have an average cell diameter ranging from about100 to 500 microns. Because the cells of microcellular plastic foams areso small, to the casual observer these specialty foams generally retainthe appearance of a solid plastic.

Microcellular plastic foams can be used in many applications such as,for example, insulation, packaging, structures, and filters (D. Klempnerand K. C. Fritsch, eds., Handbook of Polymeric Foams and FoamTechnology, Hanser Publishers, Munich (1991)). Microcellular plasticfoams have many unique characteristics. Specifically, they offersuperior mechanical, electrical, and thermal properties at reducedmaterial weights and costs.

The process of making microcellular plastic foams has been developedbased on a thermodynamic instability causing cell nucleation (J. E.Martini, SM Thesis, Department of Mech. Eng., MIT, Cambridge, Mass.(1981)). First, a polymer is saturated with a volatile foaming agent ata high pressure. Then, by means of a rapid pressure drop, the solubilityof foaming agent impregnated within the polymer is decreased, and thepolymer becomes supersaturated. The system is heated to soften thepolymer matrix and a large number of cells are nucleated. The foamingagent diffuses both outwards and into a large number of small cells.Stated somewhat differently, microcellular plastic foam may be producedby saturating a polymer with a gas or supercritical fluid and using athermodynamic instability, typically a rapid pressure drop, to generatebillions of cells per cubic centimeter (i.e., bubble density of greaterthan 10⁸ cells per cubic centimeter) within the polymer matrix.

U.S. Pat. No. 4,473,665 to Martini-Vvedensky et al., is directed to atwo-stage method for foaming thermoplastics. That patent describes afirst stage wherein a polymer is placed in a pressure vessel forsaturation with high-pressure gas. During a second stage, the polymer isheated to the Tg at a much-reduced pressure. The gas, previously forcedinto the polymer at high-pressure, foams the polymer when the polymertemperature reaches a temperature that sufficiently softens the plastic.

U.S. Pat. No. 5,684,055 to Kumar et al. discloses a method for thesemi-continuous production of microcellular foam articles. In apreferred embodiment, a roll of polymeric sheet is interleaved with agas channeling means (e.g., porous paper, gauze, mesh, woven andnon-woven fabrics) to yield an interleaved cylindrical roll. Theinterleaved roll is exposed to a non-reacting gas at elevated pressurefor a period of time sufficient to achieve a desired concentration ofgas within the polymer. The saturated polymer sheet is then separatedfrom the gas channeling means and bubble nucleation and growth isinitiated by heating the polymeric sheet. After foaming, bubblenucleation and growth is “quenched” by rapidly cooling the foamedpolymeric sheet. The '055 patent is entirely silent with respect to howa foamed polymeric sheet is to be subsequently shaped into an article ofmanufacture.

For years, customers and food vendors searched for a disposable coffeecup that was not too hot to hold, would keep the coffee warm, and didnot contaminate the coffee or our planet.

Even though they appreciated their insulating quality, consumersrejected PS (polystyrene) foam cups as too polluting and unhealthy.Major coffee vendors have used an additional costly insulating sleeve tosatisfy their customers. Many food-packaging products call for theinsulating qualities of foam without the hazards of PS foam.

Thermoforming is a conventional method for forming three dimensionalshapes from flat polymer sheets. The process heat softens the flatpolymer sheet and then vacuum or pressure forms the sheet onto a diewith the required shape. Thermoforming produces general packaging aswell as PS Foam Clamshells and solid plastic cups. When thermoforming isused to form a deep product such as a coffee cup, etc. from a flatplastic or foamed sheet, it is termed a deep draw.

As is understood by those of skill in the art, thermoforming in generalrefers to a set of related processes for producing shaped articles ofthermoplastic. Included in thermoforming are the processes of vacuumforming, pressure assisted thermoforming, high definition thermoforming,drape forming, press forming and line bending.

While advances have been made in the field of thermoformed-foamedpolymers, there remains a need for improved products and methods relatedto the manufacture of such products. The present invention fulfillsthese needs and provides for further related advantages.

SUMMARY OF THE INVENTION

In brief, the present invention in one embodiment is directed to a deepdrawn microcellularly foamed polymeric container comprising a polymericsidewall integrally connected to a polymeric base along a bottom edge.The polymeric sidewall and base are contiguous with each other anddefine a shape of an open top container. The polymeric sidewall and basehave a contiguous inner microcellular foam structure (having averagecell diameters ranging from about 5 to about 100 microns) surrounded bya smooth outer skin layer integrally connected therewith. The polymericsidewall defines a container height and a top opening, wherein the topopening defines a top opening width, and wherein the polymeric basedefines a container base width, and wherein the area defined by the topopening is greater than the area defined by the polymeric base, andwherein the ratio of the container height (h) to the top opening width(w) is greater than about 1:1 (h:w). In some embodiments, the polymericsidewall and base comprise polyethylene terephthalate (PET). In someembodiments, the areal draw ratio defined by the shape of the open topcontainer is greater than about 1.5.

In another aspect, the present invention is also directed to a methodfor making a deep drawn microcellularly foamed polymeric container froma polymeric sheet or roll. This method comprises at least the followingsteps: pressurizing the polymeric sheet or roll with a plasticizing gasunder a selected pressure and period of time sufficient to yield areversibly plasticized thermoplastic material, the plasticizedthermoplastic material being impregnated with the plasticizing gas;depressurizing the plasticized thermoplastic material to thereby desorbsome of the plasticizing gas from the plasticized thermoplasticmaterial; heating the plasticized thermoplastic material to atemperature sufficient to cause softening and foaming, wherein thefoaming generates a plurality of microcells within the plasticizedthermoplastic material; and shaping the plasticized thermoplasticmaterial into the deep drawn microcellularly foamed polymeric container,wherein the deep drawn microcellularly foamed polymeric container has awidth to depth ratio of greater than about 1:1 and an areal draw ratioof greater than about 1.5.

These and other aspects of the present invention will become moreevident upon reference to the following detailed description andattached drawings. It is to be understood, however, that variouschanges, alterations, and substitutions may be made to the specificembodiments disclosed herein without departing from their essentialspirit and scope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a solid-state gas impregnationand thermoforming system useful for making deep drawn microcellularlyfoamed polymeric containers (from an interleaved gas impregnatedpolymeric roll) in accordance with an embodiment of the presentinvention.

FIG. 2A is an enlarged and elevated side perspective view of a deepdrawn microcellularly foamed polymeric container made in the manner asillustrated in FIG. 1.

FIG. 2B is an enlarged side perspective view of the deep drawnmicrocellularly foamed polymeric container illustrated in FIG. 2A, buthaving a cut-out section that shows a cross-section of the wall of thefoamed container.

FIG. 2C is an enlarged top view of the deep drawn microcellularly foamedpolymeric containers illustrated in FIGS. 2A-B.

FIG. 3 is an enlarged side view of a wall portion of the deep drawnmicrocellularly foamed polymeric container shown in FIG. 2B.

FIG. 4 is a graph illustrating CO₂ gas concentration over time elapsedsince foaming in polymer foamed by the solid state process (i.e.,desorption time associated with the various Examples disclosed herein).

FIGS. 5 and 6 are cross-sections of polymer structures obtained beforeand after thermoforming when used with high levels of gas-inducedcrystallinity prior to thermoforming.

FIGS. 7 and 8 are cross-sections of polymer structures obtained beforeand after thermoforming when used with low levels of gas-inducedcrystallinity prior to thermoforming.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to deep drawn microcellularly foamedpolymeric containers, as well as to methods of making the same. In theseveral embodiments disclosed herein, the deep drawn microcellularlyfoamed polymeric containers are described in the context of transforminga solid monolithic sheet of a PET (polyethylene terephthalate)thermoplastic material; however, it is to be understood that othersemi-crystalline polymers such as, for example, PEEK(polyetheretherketone), PEN (polyethylene napthalate), PBT (polybutyleneterephthalate), PMMA (polymethyl methacrylate), and PLA (polyactide), aswell as various polymeric blends thereof, are contemplated and withinthe scope of the invention. In addition, and as appreciated by thoseskilled in the art, PET is understood to be inclusive of both RPET(recycled polyethylene terephthalate) and CPET (crystallizingpolyethylene terephthalate).

The present invention utilizes a solid state process of gas impregnationto enhance the performance of thermoplastic material used inthermoforming. A roll of polymer sheet is provided with a gas channelingmeans interleaved between the layers of polymer. The roll is exposed toa non-reacting gas at elevated pressure for a period of time sufficientto achieve an elevated concentration of high-pressure gas within thepolymer. If the gas is a plasticizing gas, exposure is for a period oftime required to bring about a plasticizing effect of the polymer. Thesaturated polymer sheet is then separated from the gas channeling meansand decompressed and subsequently thermoformed. In embodiments utilizingplasticizing gas, the glass transition temperature of the exposedpolymer is reduced, and therefore thermoforming may take place at alower temperature than used for thermoforming unexposed polymer.

In some applications, the invention provides foaming the polymer priorto thermoforming by creating high levels of dissolved gas during gasexposure. In some embodiments practicing foaming, bubble nucleation andgrowth proceeds spontaneously upon decompression, while in other foamedembodiments bubble nucleation and growth is initiated and enhanced byheating the polymer sheet near to or above the polymer's glasstransition temperature, thereby producing foamed polymer ready forimmediate thermoforming. In embodiments practicing foaming, theprocesses of foaming and thermoforming may be continuous. In preferredembodiments practicing continuous foaming and thermoforming, foaming isperformed by heating just prior to forming.

In another aspect, a method is provided for thermoforming foamsemicrystalline-polymers with control of the unfoamed skin thickness,independent control of the depth and percentage crystallinity of thesurface layers, as well as the core percent crystallinity. This methodalso reduces energy consumption, increases quality, and significantlyincreases production rate of thermoformed polymer foams. This methoduses high levels of gas-induced crystallinity prior to thermoforming andresults in higher allowable service temperature limits. The reversibleplastizing imbued upon the thermoplastic by the foaming gas increasesthe allowable depth of draw during thermoforming.

In yet another aspect, a thermoformed semicrystalline-polymer materialis taught, the material having an unfoamed skin of controllablethickness and percentage crystallinity as well as a foamed core with alower percent crystallinity. The “gas-impregnated thermoformed” materialallows higher formed part service temperatures than conventional throughcrystallized solid PET thermoformed parts with similar impact strength.Alternately, “gas-impregnated thermoformed” polymer allows higher formedpart impact strength and ductility than conventional throughcrystallized solid PET thermoformed parts with similar maximum servicetemperature. Further, this material increases the practical depth ofdraw or the maximum practical crystallinity level for a given depth ofdraw during a thermoforming operation. Other semi-crystalline polymersexperience similar benefits when thermoformed after being injected withcrystallizing gas.

In still another aspect, a food-to-go container is fabricated fromfoamed thermoplastic, such as PET for example. The container, which maybe used for consumer, food processors, and institutional applications,includes features for maintaining desirable properties of food such asfreshness, crispness, crunchiness, heat, cold, etc. Additionally, thematerial may be formed to withstand relatively large swings intemperature including, for example, freezer-to-oven,refrigerator-to-microwave, etc. This allows consumers and institutionsto use the packaging for processing, cooking, storage, reheating,serving, etc.

Although in principle many thermoplastic sheet materials may beconverted into foamed containers according to the present invention, PETand partially crystalline PET (C-PET) are preferred for many of thecontainers and especially food containers described herein because thosematerials are especially desirable for, and recognized by the FDA fordirect food contact. A desirable property of the process describedherein is that the foaming agents reversibly plasticize thethermoplastic, enabling the use of C-PET throughout the process ratherthan requiring it to be formed in situ during finishing. In anotheraspect, the process allows higher degrees of crystallinity in thefinished product, a property that generally increases hardness andthermal stability of the finished product. In another aspect, theprocess allows variable crystallinity through the section of productsformed therefrom, thus enabling customization of material properties.

The polymeric materials noted above may be saturated by exposure to agaseous working fluid under pressure. For example, PET or C-PET may beexposed to carbon dioxide at 250 to 2500 PSIG, and especiallyapproximately 270 to 2430 PSIG. The high pressure causes carbon dioxidegas to soak into the plastic rolls. Once saturated with gas the plasticroll is removed from the pressure vessel and proceeds to step two of theprocess. With the working fluid thus forced into the polymer matrix, thematerial may be thought of as nascent foam. Because the working fluid,in preferred embodiments, does not react chemically with thethermoplastic material, the material is not chemically altered and thusmay continue to be recycled. The working fluid does, however, reversiblyplasticize the material, reducing its glass transition temperature,T_(g), and making materials with normally high T_(g) more amenable toprocessing.

After saturation and before expansion, the material may be held for aquiescent period of time. When held at a pressure and/or temperaturelower than the pressure and temperature of the saturation step such asambient temperature and atmospheric pressure for instance, the materialis, in fact, supersaturated. During the quiescent period, which may beadjusted and may be accompanied by environmental changes such aspressure changes, temperature changes, etc. according to desiredproperties of the material, dissolved working fluid at the surfaces ofthe sheet escapes into the surrounding environment. In the case of acarbon dioxide working fluid, it is permissible to allow the escaped gasinto the atmosphere or alternatively to be recycled. By allowing thesurface working fluid to escape, the localized material at the surfaceis no longer a nascent foam and will not form a foam during subsequentprocessing. The thickness of the surface layer thus affected may beadjusted by adjusting the quiescent time or otherwise changing theenvironmental conditions. Longer times allow more gas to escape,resulting in thicker surface layers. The surface layers are useful forimproving cosmetic appearance and for enhancing abrasion and cutresistance. Generally the quiescent period may last from 10-24 hours forexample. Refrigeration of the material during the quiescent or“de-sorption” period can extend it for greater process flexibility.

The nascent foam is then expanded into a closed microcellular foam byexpanding in the presence of heat during expansion. As depicted in FIG.1, the gas-saturated plastic roll is unwound and as it is being unwoundit is fed into a heating tunnel in which it is heated. The heat causesthe plastic to soften and the absorbed gas in the plastic coalesces intobillions of small bubbles. The plastic expands in volume while retaininga surface skin of unfoamed plastic. At the exit end of the tunnel theresulting foam looks completely solid and still is a flat sheet. Whenheated to near its T_(g) the polymer matrix relaxes, thus allowing thehigh pressure dissolved working fluid to expand at the reduced pressureof the expansion apparatus. For PET, which has a T_(g) of approximately150° C., this may be done in a hot air tunnel, by surface contact, byradiation heat transfer under infrared emitters, in hot liquid, or usingother methods known to the art.

By controlling the saturation time, pressure, and temperature in thefirst stage, heavily crystallized PET surface layers are formed beforefoaming. The second stage of that process, subsequent foaming with heat,does not foam the heavily crystallized layers. Once the desired skindepth is crystallized above a threshold percentage level (about 19% forPET) the skin will not foam during the subsequent foaming operation.This limits the depth of crystallinity level that has obtained 19% ormore, to a thickness less than or equal the desired finished unfoamedskin thickness. The “desorb time” or quiescent period (elapsed timeafter removal from the high pressure tank until heating for foaming) mayalso control the depth of the unfoamed surface skin.

In the above-described process, a polymer sheet is first interleavedwith a gas channeling means to form an interleaved roll, stack ofsheets, or festoon of polymer and gas channel. Gas channeling meanspreferably consists of a layer of flexible gas permeable material. Whileporous paper sheet is a preferred material, other gas permeablematerials, such as particulate material, gauze, mesh, and woven ornon-woven fabrics, may also be successfully employed in the presentinvention.

Interleaved material is next exposed under elevated pressure to anon-reacting gas which is soluble in the polymer for a time sufficientto achieve a desired concentration of gas within the polymer, typicallyat least 0.5% by weight for PET-CO₂ systems. Exposure to pressure isgenerally carried out at room temperature (around 21 degrees C.). Highertemperatures may be employed to accelerate the rate of diffusion of thegas within the polymer, while lower temperatures may result in higherlevels of gas saturation over time. The pressure can be varied abovetank supply pressure with booster pumps. For example, the preferred tankpressure range when employing CO₂ is about 0.345 to 5.2 MPa. This can beincreased to over 8.27 MPa with a suitable booster pump. Pressures ashigh as 17.2 MPa or higher (supercritical CO₂) are usable.

The preferred gas can depend upon the polymer being treated. Forexample, carbon dioxide is the preferred gas for use in foaming PET, PVCand polycarbonate, while nitrogen is the preferred gas for use infoaming polystyrene. “Modified air”, which is atmospheric air in whichthe percentage oxygen has been reduced to 1% to 20% by reverse osmosisunder pressure, as well as pure atmospheric air, may alternatively beemployed in some embodiments.

The amount of time during which the polymer roll is exposed to gasvaries with the thickness of the solid polymer sheet, the specificpolymer-gas system, the saturation pressure, and the diffusion rate intothe polymer, and is generally determined experimentally. However,periods of between 3 and 100 hours are typically employed for sheetthicknesses of 0.25 mm to 2 mm. For example, when saturating a 0.5 mm.thick sheet of PET with CO₂, a saturation time of between about 15 to 30hours is preferred.

Following saturation of the polymer-gas permeable material sheet, thesheet is returned to normal pressure and the gas channeling meansremoved, yielding a sheet of gas impregnated polymer exhibiting theplasticizing effect, which gradually reverses as the gas dissipates fromthe impregnated polymer.

In some embodiments, the impregnated plasticized polymer may be foamedprior to thermoforming, while in other embodiments unfoamed plasticizedpolymer is thermoformed directly. In other embodiments, the plasticizedpolymer may or may not be foamed during the heating step ofthermoforming depending on gas saturation pressure, absorbed gasconcentration level, and thermoforming temperature.

For optional foaming in some embodiments, on unwinding from the gaschannel, the polymer sheet is heated above its glass transitiontemperature by drawing under tension through a heating station. Thepolymer sheet is thereby foamed in a continuous manner. After passingthrough the heating station, the polymer sheet may be drawn through acooling station, such as a cold water bath, a set of chilled rollers orsimply air, to cool the polymer and stop bubble nucleation and growth.In such embodiments, the temperature of the heating station as well asthe rate at which the polymer sheet is drawn through the heating stationand cooling station can be varied to provide sheets of varying bubblesize and density. After foaming, the polymer sheet is trimmed, yieldingfinished foamed polymer material which may then be thermoformed.

While embodiments may practice foaming simultaneously with forming, suchembodiments require additional forming time to allow the material tofoam, and may therefore be less adaptable to high throughput productionrequirements. Preferred embodiments for high throughput productionrequirements employ a heating station to heat the saturated polymer to atemperature suitable for both foaming and thermoforming, and thenimmediately thermoform the material without need of a cooling station.

A surprising and significant result of foaming gas impregnated polymeraccording to the processes described above is that the micro-cells inthe resulting polymer foam contain gas pressurized above atmosphericpressure. At thermoforming temperatures, the effect of the pressurizedgas trapped in micro-cells is to create secondary expansion of themicrocells, thereby keeping the cells from buckling or collapsing.Further, when the gas is plasticizing, the polymer at the cell walls ishighly plasticized, enhancing the effective plasticization of thepolymer yet further, thereby resulting in foamed polymers of lowerviscosity than expected at a given temperature.

Either unfoamed impregnated polymer or gas impregnated foamed polymermay be thermoformed. As discussed earlier, the temperature required forthermoforming articles from plasticized gas impregnated material isgenerally lower and often significantly lower than for the same materialwithout the plasticizing effect. For some gas/polymer systems in whichthe polymer is highly saturated with plasticizing gas, the polymer maybe sufficiently plasticized that the material may be “thermoformed” atroom temperature. Furthermore, because the viscosity of the polymer islowered by the plasticizing effect, for a given thermoforming process,greater detail and deeper “draws” are possible when thermoforming theplasticized material than is possible with material that has not beenplasticized. In some cases, such as vacuum forming with PET foam,articles may be thermoformed that cannot be thermoformed with polymerthat has not been plasticized.

Referring now to FIG. 1, a roll of flat polymer stock with porousseparator material between each roll layer (alternately a group ofsheets with a separator between each sheet) is placed into a pressurevessel for saturation. The pressure vessel is filled with CO₂ gas toallow the gas to saturate the polymer. The gas serves as a physicalblowing agent. The gas also serves to lower the polymer T_(g), therigidity, and melting temperature. The saturation time in the pressurevessel is dependent upon pressure, the desired gas-to-solids ratio, thegeometry of the roll, the thickness of the flat polymer stock, and othervariables. For example, with CO₂ gas at 40 atmospheres pressuresaturating PET at room temperature, a typical time in the pressurevessel is 190 hours. At this time, the surface crystallinity may beabove a 19% threshold that restricts later foaming. Saturation timelonger than this increasingly creates a deeper crystallization layer andalso increases the percentage of crystallization at a given depth.Saturation pressures may range from 20 to 180 atmospheres or more.

While this description is exemplified with PET, it should be recognizedthat other polymers or mixtures of polymers may be used in place of orin addition to PET. Suitable gas-polymer systems include CO₂ andPolypropylene, as disclosed in CO ₂-Assisted Crystallization ofPolypropylene for Increased Melting Temperature and Crystallinity byMitsuko Takada et al. Proceedings of Polymer Processing Society meeting,Hertogenbosh, Netherland, May 31, 1999. Other gases and pressures may beused (for example, CO₂ may be used with polyethylene, polyvinylchloride, acrylonitrile butadiene styrene, polycarbonate, polyethyleneterephthalate, and polypropylene; and N₂ gas may be used withpolystyrene). In preferred embodiments, the gas acts to reduce the T_(g)or melt temperature of the polymer.

The gas impregnation of the polymer may take place either below or abovethe gas's supercritical pressure. Pressures as high as 180 atmosphereshave been successfully used to foam polymers. Higher pressures result ina higher level of gas impregnation with a maximum of about 30% by weightof gas dissolved in the polymer.

By controlling the saturation time, pressure and temperature in thefirst stage, heavily crystallized PET surface layers are formed beforefoaming. The second stage of that process, subsequent foaming with heat,does not foam the heavily crystallized layers. Once the desired skindepth is crystallized above a threshold percentage level (about 19% forPET), the skin will not foam during the subsequent foaming operation.This limits the depth of crystallinity level that has obtained 19% ormore, to a thickness less than or equal the desired finished unfoamedskin thickness. The “desorb time” or quiescent period (elapsed timeafter removal from the high pressure tank until heating for foaming) mayalso control the depth of the unfoamed surface skin.

The term “skin” refers to the integral unfoamed polymer produced by thesemi-continuous process without regard to the crystallinity levels.After foaming, a layered structure exists that includes two surfacelayers consisting of highly crystallized

PET and an interior at a lower crystallinity level as illustrated inFIGS. 5 through 8. Immediately after foaming, the thickness of thesurface unfoamed skins may be greater than the raised crystallinitylevel layer as illustrated in FIG. 5, Case A, or may be approximatelyequal in thickness, as illustrated by FIG. 5, Case B. This previouslyfoamed sheet or roll stock may then be thermoformed in a conventionalthermoformer during which all layers may have their crystallinity levelsraised, as illustrated by FIG. 6, Cases A and B.

Crystallization also takes place during thermoforming heating andstretching operations depending on time, temperature and stress levels.Thermoforming crystallization takes place throughout the materialthickness approximately uniformly. The biaxial stresses created duringthe forming operation can also increase the crystallization at a giventemperature. U.S. Pat. No. 4,388,356 describes one method for“heat-setting” during a PET thermoforming operation (incorporated hereinby reference).

A preferred method combines gas crystallization with thermoformingcrystallization to create a controllable level of crystallinity in thesurface layers and in the core, which may be different than that of thesurface layers. For instance, the surface layer can be brought up toabout 14% during foaming and then subsequently raised to 29% while theinterior is raised from 0% to 15% during the thermoforming operation.This combination gives independent unfoamed surface layers.

Thermoforming a polymer above a threshold crystallinity level (19%crystallinity for PET without impregnated gas) before thermoformingseverely limits formability. In addition, thermoforming a polymer andthen raising its crystallinity level above threshold (19% crystallinityfor PET) increases the formed part's usable service temperature andstrength (see e.g. Thermoforming, A Plastics Processing Guide, G.Gruenwald, Technomic Publishing AG, 2^(d) Ed. 1998). It also makes theresultant part more brittle and subject to fracture at low impact.

Because of the reversible plasticizing effect of the impregnated gasdescribed herein, crystallinity may be raised higher than theconventional process limit of 19%, while still maintaining sufficientductility for thermoforming. After thermoforming, the gas graduallymigrates out of the material, being replaced by ambient gas (e.g. air),thus reversing the plasticizing effect and imparting the materialqualities normally associated with the higher crystallinity. Highertemperature stability is one such material quality that may be desirabledepending upon application.

This method may provide a multi-level crystalline structure that canhave a very high application temperature due to its greater thanthreshold level surface crystallinity while at the same time haveincreased ductility and impact resistance due to the lower levelcrystallinity in the foam core and the part of the unfoamed skin. Thissame principle applies to other gas- semi-crystalline polymer systemssuch as CO₂-polyropylene systems.

Another aspect of this process encompasses the use of thermoformingcrystallization immediately after the foaming operation as described inU.S. Pat. No. 5,223,545 and before the majority of the foaming gas hasbeen replaced by air. In a more specific embodiment illustrated in FIG.1, an impregnated roll is fed continuously through a hot-air tunnel andfoamed. Thence it feeds directly into an accumulator just ahead of thethermoformer and after most of the foaming has taken place. Theaccumulator allows continuous movement of the roll through the foamingsection while the material feed stops and starts within the thermoformerwith each thermoformer cycle (lasting typically 2 to 30 seconds).Alternatively, the roll may be stopped and started in the heatingsection with each thermoformer cycle thereby eliminating theaccumulator.

Thermoforming within minutes or within about 24 hours after the foamingoperation while the majority of foaming gas still remains in the polymerafter foaming yields a number of advantages compared to the separateprocesses performed with hours or days between them. Increasing the timebetween the foaming and forming allows more and more gas to escapebefore thermoforming. For example, 0.030″ thick APET saturated with 5MPa CO₂ at room temperature retained 95% of the impregnated gasimmediately after foaming. Twenty-four hours later it retained only 40%of the gas. In another example, C-PET, 0.028″ thick saturated under thesame conditions, immediately after foaming, still retained about 92% ofthe gas fully saturated sample. It retained only 17% of the gas of anunfoamed saturated sample after room temperature desorb of 24 hours. Thegas is beneficial for thermoforming since it lowers the T_(g), andincreases the formability of the polymer. For example, advantages ofthis new method include the following:

-   -   1. Saving energy and speeding up the thermoforming process by        not cooling the polymer to “quench” the bubble growth as        described in U.S. Pat. No. 5,684,055. Rather, immediately after        most (70-98%) of the foaming has completed (typically after 30        to 75 seconds of applied foaming heat for an approximately 0.1″        thick foam), the mostly foamed sheet is fed into a thermoformer        and heating is continued to a typical thermoforming temperature        significantly above the T_(g) for a few seconds. This saves        energy and speeds up the combined process since the time        required for cooling and a later reheating in the thermoformer        is eliminated. If most of the foaming were to take place in the        thermoformer, the cycle rate of the thermoformer and production        rate would be greatly slowed due to the required foaming time of        30 or more seconds (2 cpm). Thus by allowing most or essentially        all of the foaming to take place in a heating system separate        from the thermoformer, the thermoformer cycle rate is greatly        increased (30 CPM is typical). Thus a higher article production        rate is obtainable by creating most of the foam prior to the        thermoforming operation.    -   2. Improved part detail due to the combined        thermoforming-foaming operation. This combined operation allows        a small percentage of the foaming (2% to 30%) to take place        during contact with the thermoforming die which will create        better detail by filling in the die details.    -   3. Shortening the cycle time and saving energy to reach a given        level of crystallization after thermoforming by allowing a        higher percentage of polymer crystallization going into the        thermoformer. If a significant time is allowed to pass (more        than a day) before thermoforming the foam strip or sheet into a        product then significant amounts of CO₂ gas will have left the        foam. By thermoforming within minutes or hours after foaming,        the remainder CO₂ acts as a lubricant between polymer chains and        increases the formability of the polymer so a higher percentage        of crystallinity is allowed going into the thermoformer without        cracking problems during the foaming operation. The holding time        in the thermoformer to reach a given finished crystallinity        level is reduced by being able to start at a higher        crystallinity level without part cracking during thermoforming.    -   4. Shortening the cycle time and saving energy to reach a given        level of crystallization after thermoforming. If a significant        time is allowed to pass (more than many minutes) before        thermoforming the foam strip or sheet into a product then        significant amounts of CO₂ gas will have left the foam. By        thermoforming within minutes after foaming, the remainder CO₂        acts as a lubricant between polymer chains and increases the        propensity of the polymer toward forming crystalline structures        during the heating and foaming operations. The holding time in        the thermoformer to reach a given finished crystallinity level        is thus reduced.    -   5. The plastics lower strength due to the impregnated gas,        improves the die trim operation reducing the need for added trim        or trim-die heat.    -   6. It allows optimization of crystallinity levels in a        bi-crystallinity level structure in foamed and thermoformed        parts. The levels are optimized so that (a) the surface        crystallinity level is above a threshold level that        significantly raises the part's strength and useable service        temperature, and (b) the core's crystallinity level is just        below a threshold level where brittleness sharply increases. The        foam core after thermoforming includes a significant central        layer of low crystallinity level material with high impact        properties compared to the highly crystalized outer layers. The        resultant parts will have increased ductility and impact        resistance due to the flexible core for a given allowable        service temperature.

Further distinguishing characteristics of this invention, particularlywhen compared to the product of the U.S. Pat. No. 5,182,307, include thefollowing:

-   -   1. The thickness of the unfoamed skin is independent of the        depth from the surface of high crystallinity level material as        illustrated in FIG. 8.    -   2. The crystallinity level of the core layer after thermoforming        is not that of the unfoamed material, but is significantly        higher while still much lower than the high level surface        material.    -   3. The low-crystallinity core layer thickness is independent of        the thickness of foamed center material.    -   4. The material may be any semicrystalline polymer foamed with a        gas that acts as a solvent for the polymer and lower the        glass-transition temperature (T_(g)) of the polymer such as        polypropylene with CO₂ gas or PET with CO₂ gas or Polystyrene        with N₂.

For purposes of illustration and not limitation, the following Examplesmore specifically disclose exemplary method steps and actualexperimental results associated with the making and testing of variousdeep drawn microcellularly foamed containers in accordance with thepresent invention. In particular, various deep draw microcellularlyfoamed polymeric containers similar to the one depicted in FIGS. 2A-Cwere successfully thermoformed, successfully thermoformed to have awidth to depth ratio of greater than about 1:1 and an areal draw ratioof greater than about 1.5.

EXAMPLES Example 1 Trials

In each of the following Examples, 0.762 mm thick virgin PET wassaturated with CO₂ at 4 MPa pressure for 67.25 hours at 21 deg. C.Within 10 minutes after depressurization, the saturated material wasfoamed at 100 deg. C., yielding foamed polymer with little or nonoticeable surface skin and rough surface texture. Thermoforming ovenswere held at a constant temperature (about 550 deg. C.). The temperatureof plastic that was thermoformed therefore increased with duration ofheating. A one-sided male mold was employed, having a 2.4 areal drawratio, height 11.11 cm, top opening 8.636 cm, height to width ratio of1.29, bottom diameter of 5.842 cm, average wall angle of 6.5 degreesfrom vertical. The degassing time after foaming was varied to observethe effect of degassing on thermoforming at different temperatures offoamed objects having no significant surface skins. As the elapseddegassing time after foaming increases prior to thermoforming, the gasconcentration in the polymer decreases, as illustrated in FIG. 4.

1. Trials with 10-19 minutes degas time after foaming:

-   -   Forming pressure: 0.31 MPa. Secondary expansion in thermoform        observed in all trials.    -   Trial 1: 0.7 sec. heat time: foam broke through, no cup.    -   Trial 2: 10 sec. heat time: formed cup, some creases, good mold        detail definition.    -   Trial 3: 15 sec. heat time: blisters and bubbling—not enough        skin to keep contain secondary expansion of bubbles.

2. Trials with 2.5 hrs degas time after foaming:

-   -   Forming pressure: 0.31 MPa. Secondary expansion in thermoform        observed in all trials.    -   Trial 1: 8 sec. heat time: foam broke through, no cup indicating        not enough ductility.    -   Trial 2: 12 sec. heat time: cup formed, good mold detail        definition, some creases.

3. Trials with 23 hrs. degas time after foaming:

-   -   Forming pressure: 0.31 MPa. No secondary expansion in        thermoformer noted in any trials.    -   Trial 1: 4 sec. heat time: cup formed, poor definition, no        creases.    -   Trial 2: 8 sec. heat time: cup formed, poor definition, no        creases.    -   Trial 3: 10 sec. heat time: cup formed, poor mold detail        definition, no creases.

4. Trials with 51 hrs. degas time after foaming:

-   -   0.758 MPa forming pressure required for forming. No secondary        expansion in thermoformer noted in any trials.    -   Trial 1: 4 sec. heat time: plastic pulled out of clamp frame        when object reached a depth of about 5 cm.    -   Trial 2: 8 sec. heat time: plastic pulled out of clamp frame at        full depth, partial cup.    -   Trial 3: 14 sec. heat time: cup partially formed, plastic clamp        frame not holding plastic sheet against stretching.

Example 1 Conclusions

When thermoforming foams without thick or noticeable skin:

-   -   a. Short degas times after foaming limited heat time (foam        temperature) to too low a temperature for thermoforming—longer        times caused blistering;    -   b. The best compromise of gas concentration versus formability        (ductility) was at a few hours desorb time;    -   c. Longer degas times decreased formability. At 51 hours, a cup        could not be formed with 14 seconds of heat time due to low        ductility, even at 110 psi forming pressure, where at 23 hrs        degas time, a cup was made with four seconds of heat time at 45        psi pressure;    -   d. Secondary expansion in thermoformer increases detail.

Example 2 Trials

In each of the following examples, 0.762 mm thick virgin PET wassaturated with CO₂ at 5 MPa pressure for 26 hours at 21 deg. C. A skinof variable thickness was created by varying desorb time afterdepressurization prior to foaming. The saturated and partially desorbedmaterial was foamed at 105 deg. C for two minutes, yielding foamedpolymer with a density of 21% relative to unfoamed polymer.Thermoforming ovens were held at a constant temperature (about 550 deg.C.). The temperature of plastic that was thermoformed was thereforeproportional to duration of heating. A one-sided male mold was employed,having a 1.7 areal draw ratio, height 8.73 cm, top opening 7.62 cm,height to width ratio of 1.31, bottom diameter of 5.08 cm, average wallangle of 6.5 degrees from vertical.

Trial set 1: Foaming within 10-20 minutes of depressurization:Thermoforming was attempted within ten minutes of foaming. The cupswould not form adequately with 10-15 sec. heat time. Increasing theheating time caused the cups to warp and blister. These cups failedthrough tearing of the plastic during attempt to form. Skin did not formthat was obvious to naked eye.

Trial set 2: Desorb prior to foaming of 1.5 hours. A smooth glossy skinobserved on foamed material. All cups had 2.1 areal draw ratio.

-   -   a. Degased 38 min after foaming. 9 sec. thermoforming heat time:        cup with good surface detail. Clamp frame held plastic.    -   b. Degassed 19 hrs, 50 min after foaming, 10 sec. thermoforming        heat time.: cup poorly defined; Plastic slipped out of clamp        frame.    -   c. Degassed 99 hrs, 30 minutes after foaming, over 30 seconds        thermoforming heat time: poor cup definition. Clamp frame could        not hold plastic against higher stiffness of plastic.    -   d. Degassed 135 hours after foaming, over 30 seconds        thermoforming heat time: very poor cup definition. Clamp frame        could not hold plastic against higher stiffness of plastic.    -   e. Degassed 135 hours after foaming, 40 seconds thermoforming        heat time: cup foam walls melted through creating a spider web        effect. No useful result.

Example 2 Conclusions

-   -   a. Longer degas time required higher temperatures for forming        objects.    -   b. With more than 6.0-7.0% gas concentration, a significant        increase in formability was noted, allowing deeper draws.    -   c. With gas concentration around 0.5% by weight, little        ductility is imparted to PET.

Example 3 Trials

A number of trials were conducted with 0.889 mm thick recycled PET thatwas saturated with CO₂ at 5 MPa pressure for 40 hours at 21 deg. C. Inorder to form a noticeable skin, the polymer was depressurized andallowed to desorb CO₂ for approximately 390 minutes. Then it was foamedfor various times of 10 to 30 seconds in infrared heaters at 550 deg. C.and immediately thermoformed thereafter. A one-sided female mold wasemployed with a plug assist, having a 1.97 areal draw ratio, height11.11 cm, top opening 8.26 cm, height to width ratio of 1.31, bottomdiameter of 5.72 cm, average wall angle of 7.0 degrees from vertical.The relative density of the resulting foamed objects averaged 20%relative to the unfoamed polymer.

Example 3 Conclusion

Continuous processing from foaming to thermoforming is possible usinggas impregnated polymer, resulting in objects of relatively low density,having steep walls and height to width ratios over 1:1.

Comparing Example 1 to Examples 2 and 3, it is clear that solid integralskin adds strength, thereby allowing deeper draws, and containssecondary expansion, thereby inhibiting blister formation even at highergas concentrations. Integral skin allows use of open one sided toolingrather than the closed tooling commonly employed in prior art foamthermoforming.

In view of these Examples, it is clear that the present inventionencompasses various deep drawn microcellularly foamed containers. Forexample, and based on the data and results of these Examples and asshown in FIGS. 2A-C and 3, the present invention in one embodiment isdirected to a deep drawn microcellularly foamed polymeric container 10comprising a polymeric sidewall 12 integrally connected to a polymericbase 14 along a bottom edge 16. The polymeric sidewall 12 and base 14are contiguous with each other and define a shape of an open topcontainer. The polymeric sidewall 12 and base 14 have a contiguous innermicrocellular foam structure 20 (having average cell diameters rangingfrom about 5 to about 100 microns) surrounded by a smooth outer skinlayer 22 a, 22 b integrally connected therewith (as best shown in FIG.3). The polymeric sidewall 12 defines a container height h and a topopening 18, wherein the top opening 18 defines a top opening width w₁,and wherein the polymeric base 14 defines a container base width w₂, andwherein the area defined by the top opening 18 is greater than the areadefined by the polymeric base 14, and wherein the ratio of the containerheight (h) to the top opening width (w₁) is greater than about 1:1(h:w₁). In some embodiments, the polymeric sidewall 12 and base 14comprise polyethylene terephthalate (PET). In some embodiments, theareal draw ratio defined by the shape of the open top container isgreater than about 1.5.

While the present invention has been described in the context of theembodiments illustrated and described herein, the invention may beembodied in other specific ways or in other specific forms withoutdeparting from its spirit or essential characteristics. Therefore, thedescribed embodiments are to be considered in all respects asillustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription, and all changes that come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

What is claimed is:
 1. A deep drawn microcellularly foamed polymericcontainer, comprising: a polymeric sidewall integrally connected to apolymeric base along a bottom edge, wherein: the polymeric sidewall andbase are contiguous with each other and define a shape of an open topcontainer, the polymeric sidewall and base have a contiguous innermicrocellular foam structure surrounded by a smooth, unfoamed outer-skinlayer, each of the inner microcellular foam structure and the unfoamedouter-skin layer having a degree of crystallinity greater than zero butless than 19%, and wherein the outer skin layer's degree ofcrystallinity is greater than the inner microcellular foamed structure'sdegree of crystallinity, the polymeric sidewall defines a containerheight and a top opening, wherein the top opening defines a top openingwidth and has an area, the polymeric base defines a container base widthand has an area, the top opening's area is greater than the polymericbase's area, and the ratio of the container height (h) to the topopening width (w) is greater than about 1:1 (h:w).
 2. The deep drawnmicrocellularly foamed polymeric container of claim 1 wherein thepolymeric sidewall and base comprise polyethylene terephthalate (PET).3. The deep drawn microcellularly foamed polymeric container of claim 2wherein the inner microcellular foam structure comprises a plurality ofclosed cells having average cell diameters ranging from about 5 to about100 microns.